Timing signal generating means for a high speed printer

ABSTRACT

A timing signal generating system for use in a printer of the type in which a plurality of hammers are arranged along a print line and in which an endless belt, which supports charactercarrying slugs, moves along the print line at a speed of S inches per second, so that different characters become aligned with different hammers. The average character spacing is Pc and the average hammer spacing is Ph, where Pc/Ph X/Y, X and Y being integers, with X greater than Y. The system includes a sensor which senses the passage of each slug thereat to provide a pulse to a phase lock loop. The loop includes a voltage controlled oscillator which clocks an X-stage ring counter, with X output lines. The signals on the counter&#39;&#39;s output lines represent the timing signals. The loop includes a phase detector which produces an output to a loop filter as a function of the phase relationship between the pulses produced in response to the sensed passing slugs and the signals (pulses) on one of the counter&#39;&#39;s output lines. The filter&#39;&#39;s output controls the output frequency of the oscillator. The filter, which acts as an integrator, has a time constant chosen so that the frequency of the oscillator is equal to X.S/Pc as long as the average spacing between characters passing said sensor during any interval, equal to the filter&#39;&#39;s time constant, is equal to Pc.

tilted @tates Patnt [1 1 Lytle [451 Feb. 111, 11975 TIMING SIGNAL GENERATING MEANS FOR A HIGH SPEED PRINTER [75] Inventor: Dan R. Lytle, Granada Hills, Calif.

[73} Assignee: Data Products Corp., Woodland Hills, Calif.

[22] Filed: Jan. 17, I974 [21] Appl. No.: 434,260

[52] U.S. Cl... 101/93.14, 340/1725 [51] Int. Cl B4lj 7/08 [58] Field of Search 101/93 C, 111; 340/1725 [56] References Cited UNITED STATES PATENTS 3,303,775 2/1967 Giannuzzi l0l/93 C 3,519,893 7/1970 Schwartz 101/93 C X 3,593,658 7/1971 Artom 101/93 C 3,603,252 9/1971 Orsatti 101/93 C 3,629,848 12/1971 Gibson 340/1725 3.654.857 4/1972 Marsh 101/93 C Primary Examiner-Robert E. Bagwill Assistant Examiner-Edward M. Coven A ltorney, Agent, or Firm Lindenberg, Freilich, Wasserman, liosen & Fernandez 7' '1 V [57] ABSTRACT A timing signal generating system for use in a printer of the type in which a plurality of hammers are arranged along a print line and in which an endless belt, which supports character-carrying slugs, moves along the print line at a speed ofS inches per second, so that different characters become aligned with different hammers. The average character spacing is P and the average hammer spacing is P,,, where P,./P, X/Y, X and Y being integers, with X greater than 1. The system includes a sensor which senses the passage of each slug thereat to provide a pulse to a phase lock loop. The loop includes a voltage controlledoscillator which clocks an X-stage ring counter, with X output lines. The signals on the counters output lines repre' sent the timing signals. The loop includes a phase detector which produces an output to a loop filter as a function of the phase relationship between the pulses produced in response to the sensed passing slugs and the signals (pulses) on one of the counters output lines. The filters output controls the output frequency of the oscillator. The filter, which acts as an integrator, has a time constant chosen so that the frequency of the oscillator is equal to X'S/P as long as the average spacing between characters passing said sensor during any interval, equal to the filters time constant, is equal to P 10 Claims, 6 Drawing; Figures & 4|, 22? s coon'rag 5,, p35 a e e o e e PATEMTEUFEBI H915 3.865.029

SHEEI 1 OF 4 PQINTEQ CONTQOLLEQ 2o PHASE DET.

FILTER 12mg VCO TIMING SIGNAL GENERATING MEANS FOR A HIGH SPEED PRINTER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printer apparatus and more particularly to a system for generating timing signals for hammers used to print characters on a print medium by striking the print medium against characters on a movable character belt or chain.

2. Description of the Prior Art In one form of a high speed printer, the print mechanism consists of a type carrier device, e.g., a belt having an array of type representing different characters to be printed. Printing is done by moving the array of characters or simply the characters, relative to a print station at which a print medium is located and against which the characters are selectively brought by means of print actuators, such as print hammers. The hammers are arranged in uniformly spaced relation along the line of travel of the characters. They are designed to be individually operable by actuators to strike the print medium against appropriate characters as the latter are in motion along the print line. One example of such a printer is described in US. Pat. No. 3,303,775.

Typically, the spacing between hammers is less than the spacing between characters on the moving belt. In the above-referred to patent, the character spacing, hereinafter referred to as the character pitch, is 3/2 times the hammer spacing or pitch. Such a pitch ratio produces a mechanical operation where characters are aligned in a subcycle fashion in which characters are aligned at every third print hammer during each subcycle. Three subcycles are required for successive characters on the belt to be aligned with the same hammer. In order to insure proper printing, precise hammer actuating timing signals are needed to activate only those hammers with which the characters to be printed are aligned.

In the prior art, relatively complex arrangements are used to provide the necessary timing signals. Some of these arrangements employ timing gears which are mechanically coupled to the moving belt or to the belt moving mechanism. These arrangements do not account for small character spacing differences on the belt which are unavoidable and which are often on the order of a few thousandths of an inch. Nor do prior art arrangements provide timing signals in which the spacing error of a large number of adjacent characters is averaged out to be substantially zero. Consequently, the resulting print out is not as satisfactory as desirable.

OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a new system for generating accurate timing signals in a printer in which a train of characters is moved past the print station.

Another object of the invention is to provide a system for generating accurate timing signals for hammers in These and other objects of the. invention are achieved by a system in which the timing signals are generated by means of a phase lock loop (PLL), used to multiply the frequency at which characters on the moving belt pass by a sensing station. The factor by which the frequency is multiplied is related to the character pitch and hammer pitch ratio. The movement of characters past the sensing station is sensed to produce control signals which are supplied to the PLL. With a character pitch to hammer pitch ratio, defined as X/Y, where both X and Y are integers, X being greater than Y, the PLL includes an X-stage ring counter which provides X successive timing signals on its X output lines, during the average travel time of each character past the sensing station.

The timing signals on each line: is used to activate a different group of hammers. Each group consistsof every Xth hammer in the hammer array.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a general diagram useful in explaining the type of printer in which the present invention is used;

FIG. 2 is a diagram of hammers and characters in different alignment relationships;

FIG. 3 is a block diagram of the present invention;

FIG. 4 is primarily a partial isometric view of a character belt;

FIG. 5 is a more detailed block diagram of one embodiment of the invention; and

FIG. 6 is a multiline waveform diagram useful in explaining the embodiment shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The type of printer in which the invention finds particular use may best be explained in connection with FIG. 1. Therein, numeral 10 designates a belt, mounted on a pair of spaced drive wheels 11 and 12, adapted to be driven by a motor M. The direction of motion of the belt is represented by arrows 13. The belt supports a plurality of type elements or slugs designated Fl-FN, each of which is assumed to bear one character on. the front face thereof. Thus, designations Fl-FN can be regarded as representing characters. They are spaced around the belt as uniformly as possible. However, some spacing variations between adjacent characters, of the order of several thousandths of an inch, are unavoidable. The spacing between characters is referred to as the character pitch, designated P The belt and, therefore, the characters move at a substantially constant speed transversely across a record medium 15 and an ink ribbon 16 of well known types. A plurality of print hammers designated H1, H2, H3, etc., are arranged in uniform spaced relation along the line of travel of the characters. The hammer spacing or pitch is designated P,.. The hammers are designed to be individually operated by actuators to strike characters at appropriate times when the characters are aligned with them and thereby print the characters in the print medium 15. In FIG. 1, arrow 18 designates the hammers' flight or strike direction toward the characters.

As is appreciate with those familiar with the art, the number of characters in the sequence on the belt which are simultaneously aligned with particular hammers depends on the ratio of the character pitch P to the hammer pitch P Typically, P P,,. This aspect may best be explained in connection with FIG. 2 in which in line a, hammers Hl-H9 are diagrmmed with a pitch P, 0.100 inch. In line b, characters Fl-F7 are diagrammed with a pitch P, 0.125 inch, and with F1 aligned with H1. In FIG. 2, the character motion direction is assumed to be from right to left. The pitch ratio is 0.l25/I00 /4.

In such an arrangement at each alignment position or line-up, every fourth character is aligned with every fifth hammer. Thus, as seen from FIG. 2, when F1 is aligned with H1, F5, F9, F13, F15, etc., are aligned with H6, H11, H16, H21, respectively. As the belt is moved and the next hammer-character line-up is achieved, as represented in line c, H2, H7, H12, etc., are aligned with characters F2, F6, F10, F14, etc. The next hammer-character alignment occurs when H3, H8, H13, etc., are aligned with F3, F7, F11, etc., as shown in line d. Lines e-g show three subsequent alignments of different heads with different characters.

It is thus seen that for a pitch ratio of 5/4 during each hammer-character line-up every fifth hammer is aligned with every fourth character. It should be apparent that for the particular example as the belt moves one fifth of the character spacing P i.e., a distance of 0.l/5 0.125 inch, a different group of hammers is aligned with different characters. Thus, the hammers can be thought of as being divided into five groups consisting of H1, H6, H11, etc., H2, H7, H12, etc., H3, H8, H13, etc., H4, H9, H14, etc., and H5, H10, H15, etc., with each group being aligned with characters during a different line-up position.

In operation ignoring hammer flight time and other delays, each group of hammers is supplied with an activating timing signal when the group is aligned with characters on the belt. Which of the hammers within the group are actually activated depends of course on the characters which are aligned with them as well as the particular characters to be printed out. The manner in which the information to be printed out is stored and used are well known, as represented by the abovereferred to patent. All the circuitry necessary for the printer operation, which is known, is represented in FIG. 1 by block 20 which is designated as the printer controller. Hereinafter, only the circuitry which produces the timing signals will be shown in detail.

The basic principles of operation of the present invention may be explained in connection with FIG. 3 wherein a front portion of the belt 10 is shown. An isometric view of the belt portion is also shown in FIG. 4. The belt 10 consists of metallic slugs which extend along the width of the belt 10. An alphanumeric character is shown engraved on the front end of each slug which in operation faces the row of hammers. Every attempt is made to position the slugs 30 at equal spacings along the belt. However, some small spacing differences between adjacent slugs are unavoidable even though the average spacings of all the slugs on the belt is a constant. With one character per slug, the average slug spacing is the character pitch P, which in the particular example is 0.125 inch.

Referring back to FIG. 3 in accordance with the present invention, a magnetic sensor or pick-up is placed adjacent but spaced apart from the slugs 30. As the belt moves as represented by arrow 13, the sensor generates signals, one signalper slug. The frequency of these signals is designated f1. It depends on the belt speed and the slug or character pitch P Assuming a belt velocity of inches per second (ips) and an average character pitch P 0.l25 inch,f1 160 X l/0.l25 l6O x 8 1280 pps. Thus, the average period between signals or pulses, hereinafter referred to as the pulse period, is l/l280 0.78 ms.

If the actual character pitch P for all characters is exactly 0.l25 inch, i.e., if all the slugs are spaced exactly 0.1 25 inch apart, each of the pulses from the sensor can be used by means of a frequency multiplier to provide five signals during each pulse period of 0.78 ms. However, since small spacing differences between adjacent slugs are unavoidable, the pulses from the sensor cannot be used directly to generate the necessary timing signals.

This problem is overcome by the circuit arrangement shown in simplified form in FIG. 3. It, in effect is a phase lock loop (PLL). It consists of a phase detector 36 to which the pulses from sensor 35 are applied. As

will be pointed out hereinafter, the pulses from the sensor are first applied to an amplifier whose output is applied to a zero crossing detector. It is the pulses of the latter which are applied to the phase detector of the PLL. The output of detector 36 is supplied to a low pass filter 41 which acts as an integrator. The output of filter 41 controls the output frequency of a voltage con- .trolled oscillator (VCO) 40. The output frequency of the VCO, which is designated f2, clocks a-five-stage ring counter 42 shown having five output lines (bl-(b5. It is on these lines that the timing signals appear. As is appreciated, the frequency of the signals on each of these lines is f3 l/5f2. The signals of one of these lines such as (111 atj3 are applied to the phase detector 36.

Briefly, the frequency f2 is chosen to be 5 X fl 5 X 1280 6400 pps and therefore f3 =f1. In the absence of filter 41 any change in f1, due to small spacing differences between adjacent slugs, directly affects f2 and therefore f3. However, by including filter 41, with a sufficiently large time constant, f3 is not affected as longas the average frequency over a period, equal to the filters time constant, is equal to f1.

Assuming a time constant of 7.8 ms, f2 remains at 6.4k and f3 on each of the output lines (bl-(b5 remains at 1.280k as long as the average of the pulse periods during every 7.8 ms period is equal to 0.78 ms. Alternately stated, the filter, acting as an integrator, tends to average out the effect of errors in the slugs spacing, and thereby minimizes their effect on the frequency of the timing signals. Thus, the signals produced on lines q l-5 are not dependent on the exact duration of each individual pulse period but rather on the average pulse period over the period represented by the filters time constant. As a result, minor deviations or errors of the character (or slug) spacings do not affect the output signals on lines l-5, as long as their average spacing over several characters or slugs is equal to the character pitch of 0. 125.

In the particular example, assuming a filter with a time constant of 7.8 ms as long as the total spacing between any 10 slugs is equal to 10 X 0.125 1.25 inch, proper timing signals are produced on lines 1-5 even though among the IQ slugs the spacing between some adjacent slugs may differ from the character pitch of 0.125 inch. It was determined that very satisfactory results are achieved when the filters time constant is the range of several, e.g., five pulse periods. In an embodiment with the pulse period of 0.78 ms, a filter with a ms time constant was used.

Attention is now directed to FIG. 5 which is a more detailed diagram of the invention. As shown therein, the sensor 35 is connected to an amplifier 44 whose output is connected to a zero crossing detector 45. The latter provides a pulse 46 as shown in line a of FIG. 6. With a belt speed of I60 ips and a precise slug spacing of 0. l 25 inch, the pulse period, i.e., the period between successive pulses 46 is 0.78 ms. However, if the spacing between adjacent slugs is not exactly 0.125 inch the pulse period is other than 0.78 ms. In FIG. 6, line a, several pulse periods other than 0.78 ms are diagrammed. One is designated as 0.81 ms, representing a spacing between two slugs which is greater than 0.l25 in. and the other is 0.75 ms indicating a spacing between two slugs which is less than 0.125 in. Each pulse 46 from zero crossing detector 45 is applied through a variable delay 48 to a one-shot 50. The function of the delay 48 can be ignored for the present explanation. The function of the one-shot is to act as a pulse stretcher to produce for each pulse 46 a pulse 51 of a duration equal to one-half the pulse period 0.78 ms as shown in line b of FIG. 4. Thus, the output of the one-shot 50 is essentially a symmetrical square wave of a frequencyfl I280 pps. It is this output which is applied to the phase detector 35 as one of its inputs.

The output of detector 36 is supplied to filter 41 through an amplifier 53. In FIG. 6, line c diagrams the frequency f2 from the VCO 40, which, as seen, is five times the frequency fl of pulse 46. That is, the VCO output frequency is 6400 pps in the present example. The output timing signals on lines qSl-d of counter 42 are diagrammed on lines d-h. The signals on the 5 lines are designated by numeral 61-65. As seen from FIG. 5, output line 411 is connected to the phase detector 36 through a one-shot 67. The function of the one-shot, whose output is diagrammed on line i of FIG. 6, is to act as a pulse stretcher for pulse or signal 61 on line 1. Preferably, it stretches each signal 61 to a pulse 68 of 0.39 ms so that the output of the one-shot 67 is effectively a symmetrical square wave of a half duty cycle. The output of one-shot 67 represents f3 which is effectively at the same frequency as f1 of pulses 51 from one-shot 50.

For explanatory purposes, in connection with FIG. 6, it is assumed that when the pulses 57 and 68 are in phase, the output of the detector 36 is constant, e.g., zero. On the other hand, if the two pulses are out of phase, as represented by dashed lines 71 and 72, due to slug error spacings which are reflected by variations of the pulse periods from 0.78 ms the detector output is other than zero. However, by selecting the filter with a sufficiently long time constant, these input voltage variations are integrated so that the output voltage of the filter is effectively constant and therefore the output frequency of the VCO 40 remains constant at 6400 pps. Consequently, the frequency of the pulses on each of the output lines qbl-rbS remains constant at 1280 pps with equal time periods between pulses. Also, during each period of 0.78 ms, i.e., during the average passage time of each slug past the sensor 35 equally spaced timing pulses are provided on the five output lines of the counter 42, even though some of the pulse periods may deviate from 0.78 ms, due to slug spacing errors.

In one embodiment actually reduced to practice, the phase detector 36 was producing a zero output when the two pulses compared therein were 90 out of phase, rather than in phase. In the same embodiment satisfactory performance was achieved with a filter having a 5 ms time constant.

In the particular embodiment, an additional magnetic sensor 75 is employed. It is spaced above the slugs as shown in FIG. 4 to detect only a particular slug on which a metallic strip 76 is located. The particular slug is designated by numeral 50. As shown in FIG. 5, the sensor 75 is connected to an amplifier 82 whose output is supplied to a zero crossing detector 84. Thus, each time slug passes by sensor 75, a pulse is provided by the zero crossing detector 84. One such pulse is designated by numeral 85 in line j of FIG. 6.

The pulse 85 is supplied to a variable delay 86 whose output acts to reset the counter 42 to a particular account or state, e.g., count 1. The particular slug 80 bears a particular character, e.g.,. I. since the location of the sensor 75 with respect to the hammers is known, and the characters on the belt on either side of the particular slug 50 are known, the actual characters facing the array of hammers are known. It should be stressed that the function of sensor 35 is to sense each slug on the belt which passes by it, which results in the produc tion of pulse 46 from zero crossing detector 45. It is these pulses 46 which are used to generate the accurate timing signals in accordance with the invention. On the other hand, the sensor 75 is used to sense only a particular slug on the belt to produce the reset signal 85. If desired, several specific slugs on the belt may be selected to activate sensor 75 to provide the reset-type pulses for use on the controller 20.

In practice the sensor 75 is physically moved with respect to sensor 35 until the two are exactly apart a distance equal to an integer multiple of the slug spacing of 0.125 in. Thereafter, the two variable delays 48 and 86 which are interlocked as represented by dashed line 90 in FIG. 5, are adjusted to compensate for hammer travel time and other delays in activating the hammers .by the printer controller 20 so that timing signals supplied to each group of hammers cause the hammers to strike characters only when the latter are accurately aligned with the hammers.

Hereinbefore, the invention has been described in connection with an embodiment in which each slug 30 carries only one character. The invention, however, is clearly not limited thereto. If each slug 30 carries two characters, the average spacing between slugs is 2 X 0.125 0.250 in. for the character pitch to be 0.125 in. with two-characters per slug all that is needed is a flipflop between output line d1 of counter 42 (see FIG. 5) and one-shot 67 in order to cut the frequency of the pulses to the one-shot 67 and to detector 36 by onehalf. In general, if each slug carries 11 characters, a frequency divide-by-n circuit need be inserted between counter 42 and one-shot 67.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. For use in a printer of the type having a plurality of printing hammers aligned along a printing line, movable character means for moving characters along said print line, the hammers being equally spaced at a hammer pitch definable as P,,, and the characters being substantially equally spaced at a character pitch definable as P where P JP, X/Y, X and Y being integers with X being greater than Y, whereby as said characters are moved along said print line substantially every Yth character is aligned with every Xth hammer, a system for providing timing signals for said hammers, comprismg:

first means including sensing means for sensing the movement of said characters and for providing pulses in response thereto;

a voltage controlled oscillator for providing output signals at a variable output frequency which is a function of the input to said oscillator;

an X-stage ring counter with X output lines coupled to said voltage controlled oscillator and clockable by the output signals thereof;

a phase detector for providing an output as a function of the phase relationships between the pulses from said first means and pulses on a selected output line of said ring counter; and

filter-integrating means coupled between said phase detector and said voltage controlled oscillator and having a selected time constant for integrating the output of said phase detector to control said voltage controlled oscillator to provide said output signals at a frequency definable as Z signals per second, where Z X S/P and S is equal to the speed of said characters in inches per second along said print line and P is in inches, as long as the average character pitch of characters sensed by said first means during a period related to said time constant is equal to P 2. The system as described in claim 1 wherein P 0.100 inch and P 0.125 inch, whereby X= and Y 4.

3. The system as described in claim 2 wherein S is greater than 100 inches per second and the time constant of said filter-integrating means is of the order of several milliseconds.

4. The system as described in claim 1 wherein said movable character means comprises an endless belt, a plurality of elongated spaced apart slugs supported on said belt, each slug being adapted to support It characters at one end thereof facing said hammers, n being an integer not less than one, the average spacing between adjacent slugs being equal to n P whereby the average spacing between characters on said belt is P with the actual spacings between some adjacent slugs on said belt deviating from n P with said first means providing a pulse for each slug passing said sensing means, with the interval between adjacent pulses from said first means being a function of the actual spacing between adjacent slugs on said belt.

5. The system as described in claim 4 wherein P,, 0.100 inch, and P, 0.125 inch whereby X= 5 and Y 4, and wherein S is greater than 100 inches per second and the time constant of said filter-integrating means is of the order of at least several milliseconds.

6. The system as described in claim 5 wherein n 1, whereby the average spacing between slugs is 0.125

inch.

7. In a printer, an arrangement comprising:

a plurality of actuatable hammers. the hammers being spaced apart at a constant spacing, definable as P,, along a print line;

character moving means including drive means for moving an endless succession of character-bearing slugs along said print line at a speed definable as S inches per second, each slug bearing n characters, where n is an integer not less than one, with the slugs being spaced apart in average constant distance so that the characters are spaced apart an average distance definable as P where P /P, X/ Y, X and Y being integers with X greater than Y, with the actual spacing between some adjacent slugs deviating from said average constant distance, whereby the actual spacing between some adjacent characters deviates from F an X-stage ring counter having X output lines for providing signals on said output lines as a function of the states of said counter;

a voltage controlled oscillator coupled to said ring counter to clock the latter with its output signals at a variable output frequency in a selected frequency range, the frequency being a function of the input to said voltage controlled oscillator, said frequency range including a selected frequency which is equal to X S/P a sensor positioned adjacent the slugs of said character moving means and spaced apart therefrom for sensing the passage of each slug thereby;

circuit means coupled to said sensor for providing a pulse for each slug passing by said sensor, the interval between any two adjacent pulses being related to the actual spacing between two corresponding adjacent slugs sensed by said sensor; and

,control means including a phase detector and a filter having a selected time constant, responsive to the pulses of said circuit means and the signals on a selected output line of said ring counter for controlling the output frequency of said oscillator to equal X S/P as long as the average spacing between characters passing by said sensor suring any during equal to said time constant is equal to P 8. The arrangement as described in claim 7 wherein said circuit means include a first one-shot for providing for each slug passing said first sensor a pulse of a selected polarity of a duration substantially equal to H2 P /S, and said control means include a second oneshot for extending each signal on said selected output line of said ring counter to a duration which is substantially equal to H2 P /S, with said phase detector providing an output to said filter which is a function of the phase relationship of the output pulses of said first and second one-shots.

9. The arrangement as described in claim 8 further including variable delay means for variably delaying the supply of each pulse from said first one-shot to said phase detector.

10. The system as described in claim 7 further including a second sensor for sensing a selected one of said slugs, means connected to said second sensor for providing a control pulse each time said selected slug passes said second sensor and means for applying each control pulse to said ring counter to reset it to a selected state in response to each control pulse. 

1. For use in a printer of the type having a plurality of printing hammers aligned along a printing line, movable character means for moving characters along said print line, the hammErs being equally spaced at a hammer pitch definable as Ph, and the characters being substantially equally spaced at a character pitch definable as Pc, where Pc/Ph X/Y, X and Y being integers with X being greater than Y, whereby as said characters are moved along said print line substantially every Yth character is aligned with every Xth hammer, a system for providing timing signals for said hammers, comprising: first means including sensing means for sensing the movement of said characters and for providing pulses in response thereto; a voltage controlled oscillator for providing output signals at a variable output frequency which is a function of the input to said oscillator; an X-stage ring counter with X output lines coupled to said voltage controlled oscillator and clockable by the output signals thereof; a phase detector for providing an output as a function of the phase relationships between the pulses from said first means and pulses on a selected output line of said ring counter; and filter-integrating means coupled between said phase detector and said voltage controlled oscillator and having a selected time constant for integrating the output of said phase detector to control said voltage controlled oscillator to provide said output signals at a frequency definable as Z signals per second, where Z X . S/Pc and S is equal to the speed of said characters in inches per second along said print line and Pc is in inches, as long as the average character pitch of characters sensed by said first means during a period related to said time constant is equal to Pc.
 2. The system as described in claim 1 wherein Ph 0.100 inch and Pc 0.125 inch, whereby X 5 and Y
 4. 3. The system as described in claim 2 wherein S is greater than 100 inches per second and the time constant of said filter-integrating means is of the order of several milliseconds.
 4. The system as described in claim 1 wherein said movable character means comprises an endless belt, a plurality of elongated spaced apart slugs supported on said belt, each slug being adapted to support n characters at one end thereof facing said hammers, n being an integer not less than one, the average spacing between adjacent slugs being equal to n . Pc, whereby the average spacing between characters on said belt is Pc, with the actual spacings between some adjacent slugs on said belt deviating from n . Pc, with said first means providing a pulse for each slug passing said sensing means, with the interval between adjacent pulses from said first means being a function of the actual spacing between adjacent slugs on said belt.
 5. The system as described in claim 4 wherein Ph 0.100 inch, and Pc 0.125 inch whereby X 5 and Y 4, and wherein S is greater than 100 inches per second and the time constant of said filter-integrating means is of the order of at least several milliseconds.
 6. The system as described in claim 5 wherein n 1, whereby the average spacing between slugs is 0.125 inch.
 7. In a printer, an arrangement comprising: a plurality of actuatable hammers, the hammers being spaced apart at a constant spacing, definable as Ph along a print line; character moving means including drive means for moving an endless succession of character-bearing slugs along said print line at a speed definable as S inches per second, each slug bearing n characters, where n is an integer not less than one, with the slugs being spaced apart in average constant distance so that the characters are spaced apart an average distance definable as Pc, where Pc/Ph X/Y, X and Y being integers with X greater than Y, with the actual spacing between some adjacent slugs deviating from said average constant distance, whereby the actual spacing between some adjacent characters deviates from Pc; an X-stage ring counter having X output lines for providing signals on said output lines as a function of the states of said counter; a voltage controlled oscillator coupled to said ring counter to clock the latter with its output signals at a variable output frequency in a selected frequency range, the frequency being a function of the input to said voltage controlled oscillator, said frequency range including a selected frequency which is equal to X . S/Pc; a sensor positioned adjacent the slugs of said character moving means and spaced apart therefrom for sensing the passage of each slug thereby; circuit means coupled to said sensor for providing a pulse for each slug passing by said sensor, the interval between any two adjacent pulses being related to the actual spacing between two corresponding adjacent slugs sensed by said sensor; and control means including a phase detector and a filter having a selected time constant, responsive to the pulses of said circuit means and the signals on a selected output line of said ring counter for controlling the output frequency of said oscillator to equal X . S/Pc as long as the average spacing between characters passing by said sensor suring any during equal to said time constant is equal to Pc.
 8. The arrangement as described in claim 7 wherein said circuit means include a first one-shot for providing for each slug passing said first sensor a pulse of a selected polarity of a duration substantially equal to 1/2 . Pc/S, and said control means include a second one-shot for extending each signal on said selected output line of said ring counter to a duration which is substantially equal to 1/2 . Pc/S, with said phase detector providing an output to said filter which is a function of the phase relationship of the output pulses of said first and second one-shots.
 9. The arrangement as described in claim 8 further including variable delay means for variably delaying the supply of each pulse from said first one-shot to said phase detector.
 10. The system as described in claim 7 further including a second sensor for sensing a selected one of said slugs, means connected to said second sensor for providing a control pulse each time said selected slug passes said second sensor and means for applying each control pulse to said ring counter to reset it to a selected state in response to each control pulse. 